A Study of MAC Time Update on NTFS
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Who Dun It: A Study of MAC Time Update on NTFS Dae Glendowne 1, Christopher Ivancic 2, and David Dampier 3 Abstract —When gathering evidence from a system an investigator often relies on timestamps in order to construct a timeline of events that occurred on that system. These timestamps contain the modified, created, and accessed (MAC) times for a given file or directory. These MAC times, specifically the accessed time, can be changed by a variety of processes. This needs to be accounted for when analyzing digital evidence. In this paper we have performed an initial analysis of some processes that commonly take place on the Windows 7 operating systems using the NTFS file system and how they affect the MAC times of files. We discuss this analysis as well as some of the challenges faced with determining what exactly causes the updated MAC times to be stored on a physical disk. 1 Keywords - File System, NTFS, Accessed Time, MAC 1. INTRODUCTION Digital forensics deals with the recovery and interpretation of evidence from a digital crime scene. A key part of this process is event reconstruction. Event reconstruction examines the evidence to determine why is has certain characteristics. [2] The Modified, Accessed, and Created (MAC) times are significant forensic artifacts that can be leveraged by the examiner to help perform event reconstruction of a system. These timestamps tell when the file was created, when the file was last modified, and when the file was last accessed. This information, when combined with other temporal cues, can tell a story about the system itself. While each of these times is important, we consider the last accessed time to be the most interesting due to the manner in which it is updated. We believe it is important that the investigator be aware of how timestamps are updated as well as which user actions and processes in Windows 7 can cause these updates in order to draw more accurate conclusions from the evidence presented. The MAC times of a file can be updated in a variety of ways by numerous processes, user actions, or malicious tampering. Of these, the last accessed time is the most volatile. Many processes can affect it, sometimes without direct interaction or the realization of a user. As a system is being examined, actions and processes that may have affected timestamps needs to be taken into account. This paper is a product of ongoing research with the National Forensics Training Center (NFTC). The goal of the NFTC is to provide instruction to law enforcement of modern and relevant forensics technique. To continue providing education and training to the law enforcement we conduct research in techniques and tool development in digital forensics. We believe this work will aid law enforcement in more accurate event reconstruction in the Windows 7 environment. 2. RELATED WORK While timestamp analysis is not new in the area of digital forensics, there has not been significant research into just how MAC times are formed. What exactly is an access of a file and when does it occur? When are these accesses actually recorded in a file’s attributes on the NTFS file system? These are the types of questions that began this research and the ones that we attempt to answer in this paper. Chow et. al. did a study to determine a set of rules they could apply based on the observed MAC times for a file or group of files that would indicate a likely cause for those MAC times. The focus for their paper was on 1 Mississippi State University, 665 George Perry Street, Butler Hall, Mississippi State, MS 39762, [email protected] 2 Mississippi State University, 665 George Perry Street, Butler Hall, Mississippi State, MS 39762, [email protected] 3 Mississippi State University, 665 George Perry Street, Butler Hall, Mississippi State, MS 39762, [email protected] 2012 ASEE Southeast Section Conference the behavioral characteristics of MAC times in order to aid in event reconstruction. [4] This paper will focus on the why and how of MAC time updates, particularly the last accessed time value. 3. NTFS The New Technology File System (NTFS) is the default file system for all current versions of the Windows operating system (XP, Vista, Server, and 7). In NTFS, everything is a file including the file system administrative data. The primary data structure in NTFS is the Master File Table (MFT) and every file will have at least one entry in the MFT. [1] The MFT holds information about the files and directories in MFT entries. These MFT entries store attributes and an attribute is a data structure containing a specific type of information such as a file's filename or security information for that file. In Windows interactions with the NTFS file system take the form of reading and writing attributes for a given file. An example of an attribute is the $DATA attribute which is common to every file in the file system. This attribute stores the actual content of a file such as the text for this paper. [1] The $STANDARD_INFORMATION attribute contains the timestamp information for that file. It is from this attribute that Windows determines the MAC times for a file that are displayed to the user when they view the properties of a file. There is also a $FILE_NAME attribute that contains the MAC time information as it relates to the filename for a given file. [1] This attribute is used by directories to obtain information about the files that are contained within that directory. 4. WINDOWS I/O ARCHITECTURE In our efforts to better understand what causes updates to MAC times that are written to disk, we peeked into the architecture of Windows I/O system. This section contains a brief overview of that system. The I/O system in Windows consists of a group of components that manage the interactions between hardware devices and applications. At the center of the I/O system is the I/O manager. The I/O manager provides a framework with which I/O requests can be passed to device drivers from applications as well as other drivers. These requests take the form of I/O request packets (IRPs). An IRP is a data structure constructed by the I/O manager that contains all the relevant information for a given I/O operation such as the file to be accessed and the type of access desired. [5] The components that make up the I/O manager that we are interested in are the NTFS driver, volume manager, and the disk driver. These components work closely with the log file service, cache manager, and memory manager to implement efficient I/O processing. Figure 1: NTFS and Related Components 2012 ASEE Southeast Section Conference NTFS Driver, Volume Manager, Disk Driver - These three drivers work together to enable the reading and writing of files from the disk. When user applications and system processes need access to a file they will call the NTFS driver to locate it. The NTFS driver sends an IRP to a device object that represents a volume. The volume manager will then send an IRP (via the I/O manager) to the disk driver containing the volume reference. The disk manager will locate the file and the contents that are required will be loaded into memory. Log File Service - The log file service (LFS) is composed of several functions that exist inside the NTFS driver. These functions are used to record all metadata changes that occur in the file system for the purpose of recovery. Once the LFS has written the log file to disk, the cache manager will flush the actual metadata changes to disk.[5] Memory Manager - Part of the memory manager's responsibilities include mapping accessed files from disk into memory. The memory manager uses a structure called a section object, also called a file mapping object, to map virtual addresses to these opened files. The portion of the file that is mapped into memory (the view) may be utilized by an application to read or write the data for that file. Once a file is mapped into a process’s virtual address space in memory, the program can access the file view without performing extra disk I/O. If the process needs to access a portion of the file that is not mapped into memory then the memory manager uses it’s paging mechanism to load that view into memory. This is the same way the memory manager writes changes back to disk. When a process writes to a file view, the memory manager just pages the written pages back to disk. [5] Figure 2: Relationship between file on disk, file mapping object, and file view. [8] Cache Manager - The cache manager helps the memory manager and NTFS driver maintain efficiency for I/O operations. 5. TESTING METHODS Windows, for the sake of efficiency, does not immediately write attribute modifications to disk. To test this we took the approach of not just looking at the timestamps through explorer in the properties. We wanted to see actual system calls and determine what is being updated. This section describes our process for exploring how processes can affect the MAC times of a file. The first challenge in looking for updated MAC times was simply to view the MAC times for a file without altering them by that same action. We knew from previous experience that simply right clicking a file and checking the properties can cause a last accessed time update. Our primary method for checking this was to use the dir command from the command line with the dir /t:a parameter.